An electronic blood pressure meter based on the oscillation method comprises a cuff, a pressurization pump, a vent valve for depressurizing the cuff, a pressure sensor for detecting the cuff presure, and a micro computer (MPU).
This MPU is equipped with the functions of detecting a pulse wave component from the output signal of the pressure sensor, computing a pulse wave amplitude value from the pulse wave component and determining a systolic pressure (SYS) and a diastolic pressure (DIA) from the cuff pressure and the pulse wave amplitude value.
In this electronic blood pressure meter base on the oscillation method, a threshold value is utilized as a basis for determining the blood pressure values.
Normally, in measuring blood pressure, with an artery blocked by pressurizing the cuff, a pulse wave amplitude is detected during the course of gradually depressurizing the cuff. The pulse wave is an indication of a deformation of the artery wall and the surround tissue due to the pulsation in the internal pressure of the artery, and this is transmitted to the cuff where it is detected as a pressure fluctuation. This pulse wave amplitude value gradually increases as the cuff pressure is reduced and can be represented by a curve (envelope) which gradually increases but after reaching a maximum value diminishes in value.
Thus, a maximum pulse wave amplitude value is detected and a pulse wave amplitude value which is substantially equal to a threshold value which is a certain fraction of the maximum pulse wave amplitude value (for instance 50% of the maximum pulse wave amplitude value) is determined during the gradually increasing process of the pulse wave amplitude value. And the blood pressure when the pulse wave amplitude value has substantially coincided with the threshold value is determined as a systolic pressure. Likewise a pulse wave amplitude value which is substantially equal to another threshold value which is also a certain fraction of the maximum pulse wave amplitude value (for instance 70% of the maximum pulse wave amplitude value) is determined during the gradually decreasing process of the pulse wave amplitude value, and the blood pressure when the pulse wave amplitude value has substantially coincided with the threshold value is determined as a diastolic pressure.
In such a conventional electronic blood pressure meter based on the oscillation method, the cuff pressure corresponding to the time point when the pulse wave amplitude value is 50% of the maximum pulse wave amplitude value is determined as a systolic pressure and the cuff pressure corresponding to the time point when the pulse wave amplitude value is 70% of the maximum pulse wave amplitude value is determined as a diastolic pressure.
According to this process, it is necessary to detect the pulse wave amplitude value corresponding to 50% of the maximum pulse wave amplitude value for the purpose of determining the systolic pressure. Therefore, the cuff pressure must be raised beyond the systolic pressure at which the pulse wave amplitude value corresponds to 50% of the maximum pulse wave amplitude value. Thus, not only a considerable time is required for measurement but also congestion could be cause in blood vessels which are more periheral than the part of the artery to which the cuff is applied.
Further, because of the need to raise the cuff pressure beyond the systolic pressure, it can often happen that the cuff presurization is insufficient. In such case, the cuff pressure at the time point when the pulse wave amplitude corresponds to 50% of the maximum pulse wave amplitude value cannot be detected and the measurement has to be repeated all over again. Furthermore, because it becomes known to the person using the blood pressure meter that the cuff pressure was insufficient only after the attempt to measure blood pressure has been concluded, the patient must endure the discomfort caused by the excessive cuff pressure for an unreasonably long time.